Human milk has long been thought to protect infants from diarrheal disease by a variety of antibody and non-antibody factors. However, there are little data linking specific milk components to protection from specific enteric pathogens. Although shigellosis is one of the few enteric infections for which specific evidence exists that human milk protects infants, the data regarding mechanism(s) of protection are scant. The paucity of information is due primarily to the fact that an understanding of shigella virulence has only recently emerged. This ignorance about pathogenesis has limited the scope of investigation to studies which have focused on antibodies to the lipopolysaccharide antigens of shigella. It has been found that such antibodies are transiently present in milk. Since milk protection appears to persist throughout lactation and appears to be not type specific, antibodies to lipopolysaccharides are unlikely to be the major mechanism of protection from shigella. The purposes of this study are to define whether milk truly is protective against shigella, to determine the antibody and non-antibody factors present in human milk which may play a role in protection, and to determine which of the potentially important factors are associated with freedom from symptomatic shigellosis.